A sensor cell of an integrated microfabricated sensor commonly has a sandwich structure with a cell body of crystalline silicon between two windows, with a cavity extending through the cell body, between the windows. The sensor cell may be fabricated by etching the cavity in the silicon using a crystallographic wet etch process which produces faceted cell body walls, so that the cavity is wider at one end than at the other end.
After the cavity is etched, the first window is attached. In one approach, the window is attached to the side of the cell body with the narrow cavity end, so that the wide end of the cavity is open. A solution of alkali metal salt, such as cesium azide dissolved in water or alcohol, is dispensed into the cavity. The solvent is removed by evaporation. This approach has a problem with wicking of the solution onto the exposed surface of the cell body where the second window is attached, because the surfaces are hydrophilic, and because the cell body wall has an obtuse interior angle at the exposed surface, facilitating wicking onto the exposed surface. As the alkali metal salt precipitates from the removal of the solvent, the wicking is exacerbated by diffusion of the solution through the precipitated metal salt. The precipitated metal salt on the exposed surface of the cell body interferes with the attachment of the second window. The cavity is approximately filled with the solution, to attain a desired amount of the metal in the cavity. Increasing the concentration, to reduce the fill volume, exacerbates the wicking, and has not been effective in solving the problem. Making the cavity surfaces hydrophobic, to avoid wicking, tends to precipitate the metal salt on the middle area of the window, obscuring a signal path through the cell.
In another approach, the first window is attached to the other surface of the cell body, so that the cavity is narrower at the open end. In this approach, the solution tends to precipitate the metal salt in the center of the window. This undesired precipitation in the signal path is a result of the obtuse interior angle of the cell body wall at the surface abutting the window.